The use of polymeric material for the encapsulation of cells and tissue offers great potential for the treatment of diseases and other medical indications. Particularly useful applications involve utilizing polymeric material for encapsulating tissues or cells for transplantation into a patient in order to provide therapy. Although various techniques for encapsulating mammalian cells have been known for a number of decades and have been used in research settings, only more recently cell encapsulation technologies have been applied for the potential treatment of diseases.
Cell encapsulation methods are generally aimed at surrounding a cell or group of cells with a material barrier in order to protect the transplanted encapsulated cells from host immune rejection. The material barrier around the cells ideally allows the cells to remain viable and to function properly in order to provide therapeutic value to the host. In order to perform this function, the material that is used to encapsulate the cells, which typically includes a polymeric compound, should be resistant to biodegradation and should be sufficiently permeable to allow for diffusion of cellular waste products, nutrients, and molecules involved in cellular responses. Preferably, the material barrier is not permeable to certain host molecules, such as immunoglobulins and complement factors that could contribute to the destruction of the foreign cells.
Advances in cell encapsulation technologies have been focused on improving the permselectivity, mechanical properties, immune protectivity, and biocompatibility of the material barrier that is formed around the cells. Various micro- and macroencapsulation techniques, including microencapsulation by polyelectrolyte complexation, thermoreversible gelation, interfacial precipitation, interfacial polymerization, and flat sheet and hollow fiber-based macroencapsulation have been studied and are reviewed by Uludag et al. Adv. Drug Deliv. Rev. 42:29-64 (2000).
One commonly used method for the encapsulation of cells is the alginate crosslinking method, which utilizes polyanionic alginate and polycationic polylysine polymers. Encapsulation by the alginate method typically occurs by the crosslinking of alginate via the Ca2+ ion and the interaction of polylysine with the alginate molecules. Unfortunately, there are a number of problems associated with this approach to cell encapsulation. Such problems include the swelling of alginate microcapsules due to the presence of Ca2+ in the inner alginate core, insufficient biocompatibility due to guluronic acid content in alginate/polylysine capsules, and insufficient mechanical strength of the alginate coating. Moreover, the process of alginate encapsulation is nonspecific and can result in the formation of microcapsules that do not contain the cells or cell groups intended to be encapsulated or that contain other non-target biological materials. Due to these problems, alternative methods for cell encapsulation have been investigated.
One promising alternative to alginate crosslinking is a method termed interfacial polymerization. Interfacial polymerization has the possibility of offering all of the advantages of the alginate encapsulation method for cellular encapsulation and its therapeutic applications, although there has been little done to investigate its potential. Interfacial polymerization generally involves the formation of a layer of polymerized material, such as synthetic or natural polymerizable polymers, on the surface of a biological substrate. The formation of the layer of polymeric material is generally promoted by the activation of a polymerization initiator, which is deposited on the surface of the biological substrate, in the presence of the polymerizable polymers.
Some polymerization initiators for use in interfacial polymerization methods have been demonstrated in U.S. Pat. No. 5,410,016 and U.S. Pat. No. 5,529,914. These patents describe depositing the polymerization initiator, eosin Y, on a cell membrane and then activating the initiator to promote polymerization of a macromer solution. However, the use of eosin Y, which is a relatively nonpolar, low molecular weight light-activated initiator dye, or compounds similar to eosin, presents many disadvantages for interfacial polymerization methods and also presents potential problems to subjects receiving transplanted encapsulated cells. For example, these dyes and other similar low molecular weight compounds present toxicity problems as they can penetrate into a cell and interfere with normal biochemical pathways. If penetrated into the cell, these dyes can cause free radical damage when activated by external sources of energy. Other drawbacks arise if the dye is able to diffuse out of the formed polymeric layer, thereby producing potential toxicity to a host organism. Dyes such as eosin also tend to aggregate in aqueous solution, thereby reducing the efficiency of the encapsulation process and introducing problems with reproducibility. Finally, in view of the limited efficiency of these dyes in initiating sufficient radical chain polymerization, it is often necessary to add one or more monomeric polymerization “accelerators” to the polymerization mixture. These accelerators also tend to be small molecules which are capable of penetrating the cellular membrane and have the potential to be cytotoxic or carcinogenic. Therefore, it is also desirable to minimize the use of these accelerators. In attempts to overcome the above problems, applicants have previously introduced novel interfacial polymerization reagents and techniques (see U.S. Pat. Nos. 6,007,833 and 6,410,044; herein incorporated by reference in their entirety).
Despite these teachings, improved initiators for interfacial polymerization methods are desired. The cell surface, to which the initiator polymer is targeted, is very complex and presents a challenge for the design of initiators that function in a desired manner. For example, the cell surface contains numerous surface proteins, some of which have carbohydrate groups containing charged moieties, such as sulfated proteoglycans and glycosaminoglycans. It is desirable to design initiators that localize to the biological surface but do not affect the physiology of the cell in a negative manner. For example, improved initiators should preferably promote the formation of a polymeric layer on the cell surface in an efficient manner without triggering any detrimental cellular processes, such as signaling pathways that lead to cell death.
In another aspect, it may also be desirable to have the interfacial polymerization reagents or polymeric layers formed by the initiators impart a desired effect on the cell. For example, having encapsulated pancreatic cells that produce insulin or having encapsulated thyroid cells produce parathyroid hormone can be of value to a patient in need of such a therapy. Such action may reduce or eliminate the need for the patient to take drugs that promote such an effect in vivo.